A respirator forces gas into the lungs of a patient who is incapable of breathing normally for himself. Generally speaking, normal breathing is impaired either because of pathological problems with the patient's lungs, such as high airway resistance or lung stiffness, or because of other extra-pulmonary physiological problems, such as paralysis due to poliomyelitis, head injuries, and the like, which prevent the patient from obtaining adequate ventilation.
Prior art volume-cycled respirators ordinarily contain adjustments for the number of times per minute gas is forced into the patient's lungs and for the volume of gas delivered during a cycle. However, such prior art respirators do not adequately control the volume of gas delivered as a function of time throughout each inspiratory cycle to provide a timerelated volume-flow profile which is best suited for each patient in view of his specific pathological breathing problem. For example, prior art respirators lack precise controls for optimizing the inspiratory breath and achieving maximum possible gas distribution within the lung without undesirable premature airway pressure buildup.
Thus, there is a chance that excessive pressure can build up in the lungs of a respirator patient suffering from high airway resistance or lung stiffness problems. This could injure the patient's lungs, or cause extreme discomfort while he is using the respirator. To solve this problem, prior art respirators generally have used a relief valve for "aborting the cycle", i.e., immediately venting gas in the respirator to the room air when excessive pressure buildup is sensed. However, this procedure has the disadvantage of wasting gas which should be delivered to the patient to maintain proper ventilation.
Several known respirator systems have attempted to provide more precise controls over the air delivered to the patient. Many such respirators use "open loop" control over the respirator cycling. Typical open loop systems are described in the patents to Bell and Cox referred to above. In these systems the volume of gas delivered to the patient is measured during each cycle. When the actual volume reaches some desired fixed volume, the inspiration cycle is automatically stopped.
The patent to Jonsson et al. referred to above discloses a closed loop system for controlling respiratory cycling. The Jonsson et al. system produces a reference signal which defines a desired time-dependent flow rate of gas to be delivered to the patient. The reference signal is fed to a servo unit which includes a flow meter for generating a signal representing the actual flow rate of gas being delivered to the patient. The reference flow rate signal and the signal from the flow meter are compared to generate an error signal which controls a mechanical flow control unit which pinches down or backs off on a conduit extending from a bellows to the patient to control the flow rate of gas to the patient.